The present invention relates to a field emission cold-cathode device, a method of manufacturing the cold-cathode device, and a vacuum micro device using the cold-cathode device.
Recently, field emission cold-cathode devices using semiconductor processing technologies are being actively developed. As one representative example, a device described by C. A. Spindt et al. in Journal of Applied Physics, Vol. 47, 5248 (1976) is known. This field emission cold-cathode device is manufactured by forming an SiO.sub.2 layer and a gate electrode layer on an Si single-crystal substrate, forming therein a hole having a diameter of about 1.5 .mu.m, and forming a conical emitter for performing field emission in this hole by vapor deposition. A practical manufacturing method of this device will be described below with reference to FIGS. 17A to 17C.
First, an SiO.sub.2 layer 2 as an insulating layer is formed on an Si single-crystal substrate 1 by a deposition method such as CVD. Subsequently, an Mo layer 3 as a gate electrode layer and an Al layer 4 to be used as a sacrificial layer are formed on the SiO.sub.2 layer 2 by, e.g., sputtering. A hole 5 having a diameter of about 1.5 .mu.m is then formed in the layers 2, 3, and 4 by etching (FIG. 17A).
Subsequently, a conical emitter 7 for performing field emission is formed in the hole 5 by vapor deposition (FIG. 17B). The formation of this emitter 7 is done by vertically depositing a metal such as Mo as the material of the emitter onto the rotating substrate 1 in a vacuum. During the deposition, a pinhole diameter corresponding to the aperture of the hole 5 decreases as an Mo layer 6 is deposited on the Al layer, and finally becomes 0. Therefore, the diameter of the emitter 7 deposited in the hole 5 through the pinhole also gradually decreases to form a conical shape. The excess Mo layer 6 deposited on the Al layer 4 is removed later (FIG. 17C).
Unfortunately, the above manufacturing method and the field emission cold-cathode device manufactured by the method have the following problems.
First, the emitter is formed by a rotational deposition method in which the diameter of the pinhole corresponding to the aperture of the hole 5 gradually decreases. For this reason, the height of the emitter and the shape of the tip of the emitter vary, and this degrades the uniformity of field emission. Additionally, the reproducibility of the shape and the yield are low. This extremely increases the production cost in manufacturing a large number of field emission cold-cathode devices having uniform characteristics on a single substrate.
Further, since the tip of the emitter necessary to improve the field emission efficiency is lacking sharpness, the driving voltage is increased. This poses problems such as a reduction in the field emission efficiency and an increase in the consumption power. When a high driving voltage is used, the shape of the emitter tip readily changes under the influence of a residual gas ionized by this voltage. This also raises problems in terms of reliability and service life.
Furthermore, since the SiO.sub.2 insulating layer is formed to be thick by CVD, it is impossible to accurately control the gate-to-emitter distance which has a large influence on the field emission efficiency. This degrades the uniformity of field emission and produces variations. Also, the shorter the gate-to-emitter distance, the lower the voltage by which the element can be driven. However, it is difficult to bring the gate and the emitter close to each other with a high controllability.
Moreover, because of the properties of the manufacturing method, the ratio of the height to the base length of the emitter, i.e., the aspect ratio of the emitter is difficult to increase to 2 or more. As the aspect ratio of the emitter rises, an electric field is more concentrated to the tip of the emitter. For this reason, a high aspect ratio has a great effect of decreasing the driving voltage and the consumption power. One reason for which the aspect ratio of the emitter cannot be raised is that the aperture is gradually closed in controlling the emitter height as described previously. Another reason is that the emitter base length is almost the same as the diameter of a mask used in stepper exposure, so a base length smaller than the stepper exposure limit cannot be formed. Since this stepper exposure limit imposes limitations on the emitter base length, it brings about another problem in increasing the degree of integration of the emitters.